1. Field of the Invention
The present invention is directed to a nuclear magnetic resonance imaging system, and in particular to an apparatus for indirect measurement and control of a magnetic gradient field thereof.
2. Description of the Prior Art
In nuclear magnetic resonance tomography (NMR or MRI), gradients of magnetic flux density are required in all three directions of a Cartesian coordinate system. These gradients must follow a chronologically predetermined curved shape in all three directions in the measuring volume of the imaging apparatus. This is achieved in known installations using respective coil systems for each spatial coordinate which are each driven (excited) as needed by a power amplifier. At least in part because of the overwhelming magnitude of the static magnetic field in the imaging system, however, the gradients of the magnetic flux density cannot be measured with the required precision at the speed necessary to undertake a direct control of the magnetic flux density for a particular field. Heretofore, therefore, the current flowing through the gradient coil system was regulated. Current regulation, however, is relatively imprecise because the respective shapes of the current curve and the gradient curve deviate from one another as a consequence of eddy currents in the measuring structure. Given changes in the magnetic field, eddy currents flow, for example, in the copper foil of the radio-frequency shielding, in the metal shield of the superconducting magnet used to generate the static field, and in the gradient coil itself.
An approach for enhancing the precision of such control, despite the occurring eddy currents, is disclosed by European Application 0 164 199. As described therein, it is first determined how the gradient field is distorted by the influence of eddy currents, and it is also determined what effect those distortions have on the predetermined current value and on the actual current value, so that the affect of the eddy currents on the gradient field can thereby be compensated.
Because the eddy current influence will vary for each installation, the adjustments which must be made for eddy current compensation, i.e., the necessary setting of the predetermined value and the determination of the actual value of the current, must take place at the installed nuclear magnetic resonance imaging system. Depending upon the experience of the technician, this can be a time-consuming operation. Moreover, complete compensation is usually not possible, because the eddy currents decay with significantly different time constants. Variations in the measuring structure, particularly temperature variations, are similarly not compensated.
A magnetic resonance imaging system is disclosed in German OS 37 12 033 having an actual value measurement system which measures changes in the flux density of a gradient magnetic field, and forms an actual value by integrating these changes. The actual value, after comparison to a predetermined value, can then be used to automatically adjust the gradient field. It is still possible, however, that pronounced deviations in the actual value can occur dependent on the duration of the measuring sequences because of offset disturbances.